[rjmunro]

It often makes sense to offer a cheaper ticket e.g. I can go to Brighton via London, which is faster and easier, or I can change at Reading and Gatwick, which is harder and slower, but the trains are less busy, so it makes sense to encourage people not to go via London.

The "via" restrictions are usually dropped if there are problems on the lines allowing people who are delayed to find other routes, but this can lead to problems in one area causing overcrowding problems in another.